#pragma comment(linker, "/STACK:10000000")
#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <set>
#include <map>
#include <cstdio>
#include <cassert>
#include <string>
#include <cstring>
#define ldb long double
#define LL long long
#define fi first
#define se second
#define fill(a, c) memset(a, c, sizeof(a))
#define sqr(a) ((a) * (a))
#define nextLine() {int c = 0; while((c = getchar()) != 10 && c != EOF);}
#define getBit(mask, k) (((mask) / pw[k]) % pw[1])
#define setBit(mask, k, l) (((mask) / pw[k + 1] * pw[1] + (l)) * pw[k] + ((mask) % pw[k]))
#define debug(a) cerr << #a << " = " << a << " ";
#define debugl(a) cerr << #a << " = " << a << "\n";
#define mp(a, b) make_pair(a, b)
#define pb(a) push_back(a)
#define ff first
#define ss second 
const ldb LDINF = 9128739847123.00;
const ldb eps = 1e-9;
const int inf = 1 << 28;
const ldb pi = fabsl(atan2l(0.0, -1.0));
using namespace std;

int n, k;
char s[123123];

void load()
{
	cin >> n >> k;
	gets(s);
	gets(s);
}

void solve()
{	
	int pos = 0;
	for (; pos + 1 < n; pos++)
	{
		if (s[pos] == '4' && s[pos + 1] == '7')
			break;
	}
	if (pos + 1 == n)
		return;
	for (; k;)
	{
		if (pos % 2 == 0)
		{
			if (pos + 2 == n)
			{
				s[pos] = s[pos + 1] = '4';
				return;
			}
			else if (s[pos + 2] == '7')
			{
				if (k % 2 == 1)
					s[pos] = s[pos + 1] = '4';
				return;
			}
			else
			{
				s[pos] = s[pos + 1] = '4';
				for (; pos + 1 < n; pos++)
				{
					if (s[pos] == '4' && s[pos + 1] == '7')
						break;
				}
				if (pos + 1 == n) break;
				k--;
				continue;
			}
		}
		else
		{
			if (s[pos - 1] == '4')
			{
				if (k % 2 == 1)
					s[pos] = s[pos + 1] = '7';
				return;
			}
			else
			{
				s[pos] = s[pos + 1] = '7';
				for (; pos + 1 < n; pos ++)
					if (s[pos] == '4' && s[pos + 1] == '7') break;
				if (pos + 1 == n) return;
				k--;
				continue;
			}
		}
	}
}

#define file "b"
int main()
{
	#ifndef ONLINE_JUDGE
		freopen(file".in", "rt", stdin);
		freopen(file".out", "wt", stdout);
	#endif
	load();
	solve();
	printf("%s", s);
	return 0;
}
